1. Field of the Invention
The present invention relates to an electrical heating element for vertical installation.
2. Description of the Related Art
This type of heating element has a number of resistance elements that run vertically, which resistance elements are supported by a central rod that is fixed attached to a furnace ceiling or similar. A radiative tube is positioned outside of the resistance elements, which radiative rod surrounds the resistance elements.
Such resistance elements are used principally for the heating of furnaces in industrial use. The elements consist of molybdenum silicides or an alloy of molybdenum silicide or Fe—Cr—Al material or Ni—Cr or Ni—Cr—Fe material.
Each element consists of conducting legs that run downwards and upwards a number of times. The resistance element at the top transitions into a number of terminals, which are connected to one or several sources of power. The resistance element thus hangs from the ceiling of the furnace, and extends vertically downwards during operation. The legs are exposed during operation to severe thermal variation, as a result of the development of power in the legs. The consequence of this variation is that the individual legs of the element are bent or twisted when the temperature is changed. A number of ceramic disks, therefore, are placed along the length of the element, provided with holes, through which holes the particular legs of the element pass. The function of these disks is to maintain the separation of the legs of the element such that these do not make contact with each other. Should such a contact arise, the element will be short-circuited, resulting in destruction of the element.
The ceramic disks are held in place by ceramic rods and ceramic tubes.
The uppermost ceramic disk supports, or the uppermost disks support, the weight of the resistance elements. This takes place according to the prior art in that the legs are connected in pairs with the aid of conducting plates, which plates, depending of the actual geometry of the resistance element, rest on the uppermost ceramic disk or disks. Thus the legs run in pairs through a suspension ceramic disk and are connected at the upper surface of the ceramic disk by such a conducting plate and are supported in this way by the ceramic disk.
The uppermost ceramic disk is attached, or the uppermost ceramic disks are attached, to a central rod of a metallic material.
The uppermost ceramic disk supports, or the uppermost ceramic disks support also the weight of the ceramic items that are present.
It is required to produce long such elements. It is a wish that the heating element should have a length that exceeds 2,500 mm. At such a length, the length is such that the thermal expansion of the resistance elements can no longer be managed.
This is a result of the fact that at high temperatures and when using long elements, the mechanical load on the metallic central rod, which supports the weight of the resistance elements and the said ceramic items that ensure the relative positions of the resistance elements, becomes so great that the rod becomes deformed.
Thus, the basic problem is that the weight of the resistance elements and the necessary ceramic items becomes too great.